Teen Brain’s Ability to Learn Can Have a Flip Side

A series of separate reports at the Society for Neuroscience meeting adds new support to the idea that the remarkable adaptability of the adolescent brain can be a double-edged sword: The dramatic remodeling of the brain during adolescence holds tremendous opportunities for growth and learning but also appears to increase a teen’s vulnerability to the long-term effects of environmental influences such as stress and drug experimentation.

“It’s a paradoxical stage of development,” said Frances Jensen, a neurologist at Harvard’s Children’s Hospital, who organized a briefing on the adolescent brain. “Teens actually have superb cognitive abilities and high rates of learning and memory, because they are still riding on the heightened synaptic plasticity of childhood.”

This plasticity, the brain’s ability to rewire its connections in response to experience, opens a window of opportunity that may be unmatched at any other time period. Yet, at the same time, the white matter tracts that connect discrete regions of the brain are not yet fully mature—teens may have about 80 percent of their eventual “conduction” capacity, Jensen said. The frontal lobes, where insight, judgment, self-inhibition and impulse control are centered, are the last to connect up.

“The teenage brain has this unique blend of better-than-adult capacity in one respect and not-quite-adult levels in other respects,” Jensen said. “Teens are not just young adults.”

Learning Too Well?

One consistent finding has been that adolescents are particularly vulnerable to drug use, and that the deleterious effects of drugs of abuse may be more persistent when drug use begins during adolescence, as is common. This fits with the idea that addiction is a form of “overlearning”—because the adolescent brain is still rapidly remodeling its circuits and learns faster, it may become addicted more strongly.

Heather Brenhouse and colleagues at Harvard’s McLean Hospital seem to have shown just that. They reported new data suggesting that teenagers may form stronger-held addictions, have a tougher time quitting drugs, and be more susceptible to relapse once they have quit.

Brenhouse’s research was driven in part by the desire to understand why experimentation with drugs during adolescence leads to a higher risk of addiction than at any other age. It is well established that drug addicts crave drugs when they are in an environment similar to that in which they previously used drugs or are exposed to drug “cues,” such as paraphernalia or people associated with drug use. They also are more likely to relapse under such conditions. The McLean Hospital group wondered if this drug-environment memory might be even stronger in adolescents.

To test their hypothesis, the researchers gave adolescent and adult rats repeated access to cocaine in a particular environmental setting, essentially training the animals to associate that setting with the drug. After each drug exposure, the animals were freely allowed to return to the drug-associated environment or go to another area. Overall, adolescent rats showed a stronger preference for returning to the area associated with drug use.

The researchers then asked how long it would take an animal to no longer actively seek the drug—to “extinguish” the drug use. Adolescents took 50 percent longer than adults to kick the habit. In terms of susceptibility to relapse, the data had an odd twist: there was no age difference among animals that had become addicted on very high doses of cocaine, but on low doses, adolescent rats were much quicker to relapse. “This tells us that the adolescents may be more sensitive to drug-cue associations,” Brenhouse said.

The implication is that adolescents may require novel methods of addiction therapy, the researchers said, and effective early intervention could prevent a lifetime of recurring drug addiction.

The Anti- Social Drug

Ecstasy is sometimes called “the love drug” because of its purported ability to loosen social inhibitions, but work by Jean Di Pirro and colleagues at Buffalo State College and the University at Buffalo suggests that the long-term effects of the drug may be quite the opposite.

Di Pirro studied the effects of MDMA, the primary psychoactive ingredient in Ecstasy, in adolescent rats. Researchers exposed rats to six different “binges” of MDMA over a 30-day period, then eavesdropped on their behavior via closed-circuit television.

The animals were given a choice of accommodations, one that allowed close interaction with other rats and another where they could remain in isolation. Rats are normally very social creatures; interacting and playing with other rats is a common characteristic. Yet after just one binge of MDMA, the rats showed dramatically less play behavior, an effect that ntensified with each additional drug binge.

In terms of social contact, rats under the influence of MDMA showed no difference after the first binge, but after repeated binges, they spent less time interacting. This reduced social interaction lasted into adulthood, long after the last drug exposure. A battery of neurochemical and physiological tests suggested a possible underlying reason: decreased levels of serotonin and oxytocin, two neurotransmitters associated with social behavior.

“We think this means that repeated exposure to MDMA leads to very dramatic, long-lasting changes in social behavior, and corresponding changes in neurotransmitters that mediate this behavior,” Di Pirro said. Social cognition, an advanced set of skills that involve “reading” the intentions of others and reacting appropriately, is mediated by the prefrontal cortex, which is the last part of the teen brain to fully develop.

“For humans, the implication is that repeated binge use of Ecstasy may impair social contact in adulthood, either by disrupting social interactions during adolescence or by inducing long-term changes in the brain,” Di Pirro said.

Stressed Out for Life?

If drug use during adolescence can cause brain changes that persist into adulthood, what about stress? Might chronically stressed-out teenagers be setting themselves up for psychological disorders later on? New research by Barnard College neuroscientist Russell Romeo and neuroendocrinologist Bruce McEwen of Rockefeller University offers some clues.

The researchers started with four groups of adolescent male rats: a control group and three others that were exposed to varying degrees of stress, mild to moderate. When the animals reached adulthood, the researchers performed behavioral and physiological tests to examine the effects.

“Remarkably, the animals that were exposed to any kind of stress during puberty showed an increase in depressive-like behavior as adults,” Romeo said. They all also had elevated levels of corticosterone, a primary stress hormone. In contrast, animals exposed to the same stressors after puberty had no latent behavioral or physiological effects.

The experiments underscore how stress during puberty can affect psychosocial functioning in adulthood and lend further support to other recent studies that suggest that pubertal stress may contribute to later susceptibility for psychological disorders. “Adolescence may be a period of particular sensitivity to stress,” Romeo said.

The bottom line is that “drugs and stress affect the teen brain differently,” Jensen said. “Superimposing these kinds of environmental insults on that particular period of brain development might seriously compromise the individual’s vulnerability as an adult. We need to think carefully about the environment we’re putting our teenagers in.”